We consider delivering interactive dramas. A viewer interacts with a contents provider by answering multiple-choice questions and the answers to these questions influence the plot of delivered story. All possible plots can be represented by a directed graph such that every plot corresponds to some path of the graph. A delivery should be controlled according to the directed graph such that each viewer's history of answered choices forms a path of the graph. On the other hand, because some character of a viewer is known to a contents provider from his history of choices, a viewer tries to prevent even a contents provider from linking choices made by him. In this paper, we introduce unlinkable delivery for an interactive drama and propose such a delivery system for interactive dramas that viewer's choices are unlinkable and delivery is controlled according to the directed graph.

Data retrieval is used to obtain a particular data item from a database. A user requests an item in the database from a database server by sending a query, and obtains the item from an answer to the query. Security requirements of data retrieval include protecting the privacy of the user, the secrecy of the database, and the consistency of answers. In this paper, a data retrieval scheme which satisfies all the security requirements is defined and an efficient construction is proposed. In the proposed construction, the size of a query and an answer is O((log N)2), and the size of data published by the database server when the database is updated is only O(1). The proposed construction uses the Merkle tree, a commitment scheme, and Oblivious Transfer. The proof of the security is given under the assumption that the used cryptographic schemes are secure.

The BGPsec protocol, which is an extension of the border gateway protocol (BGP) for Internet routing known as BGPsec, uses digital signatures to guarantee the validity of routing information. However, the use of digital signatures in routing information on BGPsec causes a lack of memory in BGP routers, creating a gaping security hole in today's Internet. This problem hinders the practical realization and implementation of BGPsec. In this paper, we present APVAS (AS path validation based on aggregate signatures), a new protocol that reduces the memory consumption of routers running BGPsec when validating paths in routing information. APVAS relies on a novel aggregate signature scheme that compresses individually generated signatures into a single signature. Furthermore, we implement a prototype of APVAS on BIRD Internet Routing Daemon and demonstrate its efficiency on actual BGP connections. Our results show that the routing tables of the routers running BGPsec with APVAS have 20% lower memory consumption than those running the conventional BGPsec. We also confirm the effectiveness of APVAS in the real world by using 800,000 routes, which are equivalent to the full route information on a global scale.